The invention disclosed herein is generally related to high frequency induction plasma tubes, and more specifically to a device for enabling solid and liquid samples to be introduced into a plasma generated by a high frequency induction plasma tube. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
High frequency induction plasma tubes are well known for producing high temperature gaseous plasmas. Such plasmas are useful in a number of practical applications including high temperature spectroscopic studies and preparation of microcrystalline refractory materials.
Induction plasma tubes known in the art generally include an electrical induction coil surrounding an enclosure which contains a gas. The coil is connected to a source of high frequency (400 kHz to 5 MHz) electrical power. Typically, enclosures consist of a quartz tube located inside the coil. Argon is a commonly used ionizable gas. Upon application of power to the induction coil the gas is ionized, producing a central core of hot gaseous plasma inside the enclosure.
At low power levels the plasma is concentrated in the center of the enclosure reducing the danger of heat damage to the enclosure walls. At high power levels, however, the plasma core is both hotter and larger in diameter. As a result, the quartz enclosure is easily damaged by the plasma, which typically attains temperatures higher than 10,000.degree. C. This problem is aggravated by the fact that the plasma is subject to magnetic and electrical instabilities that cause it to fluctuate in position and occasionally contact the enclosure walls. Operation at high power levels also results in the emission of intense ultraviolet radiation from the plasma, which ionizes the air around the enclosure and may result in electrical arcing in the induction coil. These adverse effects have been overcome by the use of internal water-cooled shields as described and disclosed in U.S. Pat. No. 4,431,901 issued on Feb. 14, 1984, the teachings of which are hereby incorporated by reference herein.
By using a thick segmented shield shaped in cross section to occlude line-of-sight transmissions of light, it is possible to get induction heating of the plasma because a current is induced around each of the individual segments. Without occluding line of sight transmissions of ultraviolet radiation from the plasma, the air around the windings is ionized which promotes arcing between coil turns. A counterflow cooling system is used to cool the individual segments. Such an improved shielding system makes it possible to maintain a stable plasma at temperatures on the order of 10,000.degree. C.
One problem, not addressed by the above-referenced patent, is the substantial difficulty of introducing a sample into the plasma. Solid materials placed in the plasma reach a maximum temperature much lower than that of the plasma. Consequently, introducing a sample by placing it in a crucible within the plasma is unsatisfactory. The induction field is too weak to support significant sample vaporization. Furthermore, such an arrangement lacks sufficient operator control over such important operating parameters as crucible temperature, plasma pressure and temperature, and plasma gas flow rate. Any functioning analytical system requires close control over these parameters, and the subject invention provides a practical solution to the control of these process parameters.